Method of curing rubbery fluoroelastomers



Unite Sttes Patent O METHOD OF CURING RUBBERY FLUOROELASTOMERS Lewis Dewey, Mishawaka, Ind., assi gnor to United States Rubber Company, New York, N.Y., a corporation of New Jersey No Drawing. Filed Mar. .19, 1958, Ser. No. 722,374

2 Claims. (Cl. 26087.7)

I This invention relates to a method of curing fluoroelastomer polymeric materials wherein an amine curing agent is used. More particularly my invention relates to a method of curing such fluoroelastomers as rubbery copolymers of vinylidene fluoride and hexafluoropropylene and/or rubbery copolymers of vinylidene fluoride and monochlorotrifluoroethylene.

The rubbery copolymers of vinylidene fluoride and hexafiuoropropylene or monochlorotrifluoroethylene are commercial materials that have utility in high temperature applications. This is particularly so in the area of fuel cells. With the advent of high speed and high altitude aircraft the requirements for fuel cells have been steadily increasing, so that a material capable of withstanding such temperatures as 350 550 F. for prolonged periods is desired.

While rubbery copolymers of vinlylidene fluoride and hexafluoropropylene or monochlorotrifluoroethylene are potentially valuable materials for the above and similar applications, they are somewhat disadvantageous in that a considerable time is required for a satisfactory cure. Prior art teachings indicate that a satisfactory cure requires a time of the order of twenty-nine hours. The disadvantages of such a considerable length of time are obvious when considering production requirements.

Further, it is customary to use a cardboard form in the manufacture of fuel cells. The fluoroelastomer is coated on this cardboard form. However, a 29 hour cure is not feasible because the form will not withstand the high temperatures required for the cure. Thecardboard tends "to char and disintegrate, which results in a fuel cell of improper dimensions and contour, as well as slippage of lap seams, fittings, etc.

Accordingly, it is an object of my invention to develop a considerably more rapid cure for these rubbery fluoroelastomers. Another object of my invention is to obtain a rapidcure, yet not to sacrifice any of the desirable propcities which were hitherto obtainable by use of the much longer prior art cures. A further object of my invention is to develop a rapid cure whichwill result in a fluoroelastomer having increased tensile strength and resistance to high temperature aging, yet Will retain its original physical properties of high tensile strength, elongation,

etc. possessed prior to heat aging. will become apparent hereinafter.

By fluoroelastomers which are rubbery copolymers of Additional objects al., 49 Ind. and Eng. Chem. 1687 (October 1957), the contents of which are incorporated herein by reference.

' Rubbery copolymers of vinylidene fluoride and hexafluoropropylene have been commercially produced by Du Pont Co. Its product, produced by conventional commercial methods, is known as Viton A. The molecular weight of the copolymer may vary, and typically is Within the range of fifty thousand to sixty thousand. More recently copolymers have been produced having much higher molecular weights of from 150,000 to about 200,000.

Fluoroelastomers which are rubbery copolymers of monochlorotrifluoroethylene and vinylidene fluoride are a commercially available type of synthetic rubber (sold under the trade name Kel-F) described, for example, in an article by N. E. Conroy et 'al. entitled Kel-F Elastomer Properties, Compounding, vulcanization and Fabrication, appearing in the January, 1955 issue of Rubber Age, pages 543-550, as well as in the article titled Compounding Studies of Kel-F Elastomer by Grifiis and Montermoso, appearing in the July 1955 issue of Rubber Age, pages 559-562. This elastomer is a saturated fluorocarbon polymer containing more than 50% fluorine by Weight. Methylene groups are incorporated in the highly fluorinated polymer chain, thereby rendering the otherwise normally rigid chain elastic. The polymer chain is believed to consist essentially of CH CF and CFCl groups. X-ray diagrams show that this polymer is amorphous at temperatures as low as ---40 C. On stretching to 300%, typical fiber diagrams are observed, indicating susceptibility to orientation and crystal formation. The fluorocarbon polymer has good thermal stability, as shown by the fact that there is no evidence of chain scission or halogen loss after prolonged exposure at 440 F. Typical commercial polymers of this type contain monochlorotrifluoroethylene and vinylidene fluoride in about equal amounts. Other monomer ratios, such as monochlorotrifluoroethylene and 40% vinylidene fluoride, or 95% monochlorotrifluoroethylene and 5% vinylidene fluoride are also used.

Strongly basic, primary and secondary aliphatic polyamines are satisfactory in curing the above fluoroelastomers. Typical of such amine curing agents are triethylene tetramine, tetraethylene pentamine, Trimene Base 1 (triethyl trimethylene triamine) and hexamethylene diamine. Optimum amine concentrations range from 1 to 6 parts of amine per 100 parts of fluoroelastomer.

My invention is based on the unexpected discovery that if the amine cure of the fluoroelastomer is. carried out in the presence of ammonia the vulcanization can be effected rapidly without detriment to the physical prop erties, and with positive enhancement of the tensile strength.

The vulcanization may be carried out in. an oven or autoclave at elevated pressure in an atmosphere of air and ammonia. The curing conditions will vary somewhat, depending on the exact properties and degree of cure desired in the final article, the size of the article, etc.

' In general, useful cures are obtainable over much the same 0 yinylidene fluoride and hexafluoropropylene I mean co polymers containing at least 30 to 80 percent of vinylidene fluoride and correspondingly from 70 to 20 percent of uhexafluoropropylene. It is possible to prepare copolymers containing hexafluoropropylene in any desired amount up to the theoretical maximum of 70% by weight. The properties of this kind of copolymer and methods for producing it are more fully described in Vinylidene Fluq-oride-Hexafluoropropylene Copolymer, by S. Dixon et time and temperature ranges as may be employed in ordinary rubber vulcanization. The temperature may range from about 150 to 350 F. The higher the temperature, the less is the time required for the cure. The amount of ammonia present in the air-ammonia atmosphere may vary from 4% to by volume. A preferred range is from 4 to 33% ammonia by volume.

The following examples will illustrate and compare my invention and the result obtained by it with the prior art. All parts are by weight unless otherwise indicated.

Registered trademark of Phillips Chemical Co. for a med1um-abrasion furnace black.

3 EXAMPLE 1 Preparation of a batch using Viton A This gum was mixed on a rubber min with other ingredients to provide the stock A given below.

STOCK A. Viton A s- ---a-.. Philblack A nz.

Magnesium oxide Hexamethylene diamine carbama'teas; 1.25

Total 126.25

I The fiuotoaasmm'er is milled at a mill tau temperature o'f120-150 F.

EXAMPLE 2- Gating according to' the" prior art A. 126.25 parts of Stock'A were placed'iii a heater" and subjected to an air'pressure et re at a pressure of 40 p.sli. and a temperature of 275 F. for one hour.

, After this preliminarycuring a post cure was given as follows one hour at 212 F., one hour at 250 F., one new at 300 F., one hour at 350 F., and twenty-four hours at 400 F. The post curing was carried out in a circulating air oven; the temperature being brought up to 400 F. in a step-wise fashion as shown above. The total time for cure was twenty-nine hours.

B Alternatively, the Stock A formulation of Example 1' is" molded for thirty minutes at 275 F. in-a press and then post-cured for twenty-eight hours in precisely the samemanner as the post-cure" above.-

EXAMPLE 3 Applicants ammonia-air care A. The Stock A formulation of Example 1 ispla ced in a dry heater that has been pro-warmed to 160 F. The temperature of the heater for the succeeding 5 /2" hour period is adjusted as follows:

30 minutes temperature held at 160-180 F. 1 hourtemperature gradually raised to 230 F. l hourtemperature gradually raised to 262 F.

30 minutestemperature gradually raised to 276 F.

'1 hour and forty minutestemperature held constant at 2-7 6 F. 40 minutes heater permitted to cool down. 10minutes-pressure bled to zero.

Total time 5 /2 hours.

The pressure is gradually raised to 45 p.s.i. during which time ammonia is metered inuntil the atmosphere is 8% A B. Alternatively, the Stock A formulation of Example 1 is molded for 30 minutes at 275 F. in a press and then subjected to the same 5 /2 hour'cure of Example 3A.

The following table gives comparative properties for the prior arts twenty-nine hour cure (Example 2) and applicants five and one-half hour cure (Example 3).

TABLE 1 Tensile Elonga- Hardness (p.s.i.) tion (Shore) (Percent) Prior Art Cure:

A. Air Pressure Cure at 275 F. and

40, p.s.l. followed by 29 hour post-cure. 1;, 512 340' Aged 50 hours at 550 F 1, 232 300 85 Aged 100 hours at 550]? 984 215 B. Molded 30 mins. at 275 F. followed by 29 hour pasteure... 1, 794 230 78 Aged 50 hours at 550 F 1, 440 227 81 Aged hours at 550 F 1,298 160 85 Applica'nts five an lone-half hour cure:

A. Directlyafter Curing... 2, 195 360 83 Aged 50h0urs at 550 F 1,911 290 87 Aged 115 hours'at'550 1', 618 80 97 B. Moldin 30 mills. at 275,F. iollowe by 5% hr. post-cure. 1,783 250 78 Aged 50 hours at 550 F. 1; 533 175 82 Aged 100 hours at 550 F 1, 493 90 Considering the data given above, the comparative results are striking. It is to be noted that in every instance the tensile: strength ishigherwhen my cure is used as op} posed to the priorarts twenty-nine hour cure. Further, the hardness is approximately the same in both instances, and the elongation doesnot sufier' appreciably. Most portantly, the time for the vulcanization has been decreasedsign-ificantly, my required time being less than 20% of the time as taught by the prior art.

4 Preparation of batch using KeI-F The rubbery copolymer of vinylidene fluoride and monochlorotrifluoroethylene employed in this example, andexamp es 5 and 6, was a commercial'material known as Kel-F 3700, made by the Minnesota Mining & Manufacturing Company. Kel-F 3700 is, by weight, 30% monochlorotrifluoroethylene and 70% vinylidene fluoride and has thefollowing properties:

Specific gravity 1.85. Percent fluorine Greater'than 50%. C0101 V Solubility e.. Ketones, acetates, ethe i'js. Storage stability Excellent This gum was mixed on a rubber mill-with other ingrediexits to provide the Stock B given below.

7 I W STOCK B p Kel-F 3700 100 Philblack A 20v Magnesium oxide 5 Hexamethylene diamine carbamate" 1.25

Total 126.25

The fiuoroelastomer is milled at a mill roll temperature of -150" F. V

Caring according to the prior art A. 126.25 parts of Stock B were placed in a heater and subjected to an air pressure cure at a pressure of 40 p.s.i.

, and a temperature of 275 F. for one hour.

EXAMPLE 6 Applicants ammonia-air cure A. The Stock B formulation of Example 4 is placed in a dry heater that has been pre-warmed to 160 F. The temperature of the heater for the succeeding hour period is adjusted as follows:

30 minutestemperature held at 160-180 F. l hourtemperature gradually raised to 230 F. 1 hour-ternperature gradually raised to 262 F. 30 minutes-temperature gradually raised to 276 F. 1 hour and 40 minutestemperature held constant at 276 F. 40 minutesheater permitted to cool down. minntespressure bled to zero.

5 /2 hours-total time.

The pressure is gradually raised to 45 psi. during which time ammonia is metered in until the atmosphere is 8% ammonia by volume. The time required to introduce the ammonia and raise the pressure is about one hour. The above conditions are maintained for about 4 hours, after which the oven is gradually cooled down and the pressure gradually reduced to atmospheric.

Total time: 5 /2 hours.

The following table gives comparative properties for the prior arts twenty-nine hour cure (Example 5) and applicants five and one-half hour cure (Example 6).

The vulcanizate obtained by virtue of my cure is useful for lining tanks and the like used for storing, transporting, or processing solvents and other chemicals, such as fuel cells for gasoline, etc. Thus, my air-ammonia cure is very valuable in any application wherein a fluoroelastomer is to be cured by an amine.

It should be noted that the use of my air-ammonia atmosphere in conjunction with fluoroelastomers to be cured with an amine permits the use of lesser amounts of the amine curing agents than heretofore. Thus, with air-ammonia atmosphere I have found that the high temperature aging properties of a fluoroelastomer are improved by using a lesser amount of amine curing agent The above two Stocks, C, and D, were given the applicants cure (5 /2 hours) and further heat aged for 50 hours at 550 F. After heat aging the following physical properties were noted:

Stock 0 Stock D Property Having thus described my invention, what I claim and desire to protect by Letters Patent is:

l. A method of curing a fluoroelastomer selected from the group consisting of rubbery copolymers of vinylidene fluoride and hexafiuoropropylene and rubbery copolymers of vinylidene fluoride and monochlorotrifluoroethylene in the presence of an amine curing agent comprising subjecting said fluoroelastomer under positive pressure in excess of one atmosphere to an air-ammonia atmosphere at a temperature of from 150 to 350 F., said ammonia being at least four percent by volume of said atmosphere.

2. A method of curing a fluoroelastomer selected from the group consisting of rubbery copolymers of vinylidene fluoride and hexafluoropropylene and rubbery copolymers of vinylidene fluoride and monochlorotrifluoroethylene in the presence of an amine curing agent comprising subjecting said fluoroelastomer under positive pressure in excess of one atmosphere to an air-ammonia atmosphere at a temperature of from 150 to 350 F., and ammonia being from 4 to 33% by volume of said atmosphere.

References Cited in the tile of this patent UNITED STATES PATENTS Hardy Dec. 8, 1952 West May 21, 1957 OTHER REFERENCES Bevilacqua: Cross-Linking of Latex Rubber, Science, vol. 123, January-June 1956, pages 1123 and 1124.

Dixon et al.: Vinylidine Fluoride-Hexafluoropropylene Copolymer, I.S.E.C., 49, #10, October 1957, pages 

1. A METHOD OF CURING A FLUOROELASTOMER SELECTED FROM THE GROUP CONSISTING OF RUBBERY COPOLYMERS OF VINYLIDENE FLUORIDE AND HEXAFLUOROPROPYLENE AND RUBBERY COPOLYMERS OF VINYLIDENE FLUORIDE AND MONOCHLOROTRIFLUOROETHYLENE IN THE PRESENCE OF AN AMINE CURING AGENT COMPRISING SUBJECTING SAID FLUOROELASTOMER UNDER POSITIVE PRESSURE IN EXCESS OF ONE ATMOSPHERE TO AN AIR-AMONIA ATMOSPHERE AT A TEMPERATURE OF FROM 150* TO 350* F., SAID AMMONIA BEING AT LEAST FOUR PERCENT BY VOLUME OF SAID ATMOSPHERE. 